The present invention relates in general to RF communications systems and, in particular, to the use of RF repeaters to combat interference caused by a non-collocated radio in code division multiple access (CDMA) applications within personal, cellular, and other mobile communications systems (PCS, cellular, mobile).
Throughout the world, certain radio frequency (RF) bands have been allocated for various types of communications, including personal communications system (PCS), cellular, and other mobile applications. In the United States, the Federal Communications Commission (FCC) has allocated frequency bands in the range of 824-849 and 869-894 MHz; and 1850-1910 and 1930-1990 MHz for such applications. Currently, the 824-849 and 869-894 MHz bands are used for mobile cellular communications and the 1850-1910 and 1930-1990 MHz bands are used for PCS applications. Foreign countries have also allocated certain frequencies for cellular applications, including Japan (870-885; 925-940 MHz), England (917-950; 872-905 MHz), Scandinavia (463-467.5; 453-457.5 MHz), Germany (461.3-465.74; 451.3-455.74 MHz), etc. Additionally, Europe has allocated a separate band (890-915; 935-960 MHz) for digital cellular applications in the Global System for Mobile (GSM) communications system (GSM is a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA)).
In the United States mobile cellular band, the frequency band is divided into two separate bands: A and B bands, with each band including 25 MHz bandwidth. The A band occupies 824-835 MHz and 845-846.5 MHz for subscriber station transmission and 869-880 MHz and 890-891.5 MHz for base station transmission. The B band occupies 835-845 MHz and 846.5-849 MHz for subscriber station transmission and 880-890 MHz and 891.5-894 MHz for base station transmission. In addition, the A and B bands are geographically provisioned by the FCC. The A and B bands have been allocated by the FCC to allow one service provider to occupy the A band and a different service provider to occupy the B band, thus allowing for some semblance of competition in the marketplace for mobile cellular communication providers within a given geographic area. Within each service provider""s band and geographic area, the service provider may utilize any type of technology including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), or combination thereof.
In the United States PCS band, the frequency band is divided into six separate bands: A, B, C, D, E and F bands, with bandwidths totalling 30 MHz, 30 MHz, 30 MHz, 10 MHz, 10 MHz, and 10 MHz, respectively. The A band occupies 1850-1865 MHz for subscriber station transmission and 1930-1945 MHz for base station transmission. The B band occupies 1870-1885 MHz for subscriber station transmission and 1950-1965 MHz for base station transmission. The C band occupies 1895-1910 MHz for subscriber station transmission and 1975-1990 MHz for base station transmission. The D band occupies 1865-1870 MHz for subscriber station transmission and 1945-1950 MHz for base station transmission. The E band occupies 1885-1890 MHz for subscriber station transmission and 1965-1970 MHz for base station transmission. The F band occupies 1890-1895 MHz for subscriber station transmission and 1970-1975 MHz for base station transmission. The A, B, and C bands (MTA) are currently provisioned for use in metropolitan areas, while the D, E, and F bands (BTA) are provisioned for use in areas less populated. In addition, each of these sets of bands is geographically provisioned by the FCC. The A, B and C bands have been allocated by the FCC to allow one service provider to utilize one of the bands with different service providers utilizing the remaining bands. Likewise for the D, E, and F bands. Again, this allows for competition in the marketplace for PCS communication providers within a given geographic area. Within each service provider""s band and geographic area, the service provider may utilize any type of technology including FDMA, TDMA, CDMA, or combination thereof.
Frequency division multiple access (FDMA) technology utilizes narrow and discreet channels within the frequency band. Different signals are assigned different frequency channels. Interference to and from adjacent channels is limited by the use of bandpass filters which pass the signal energy within the narrow channels while rejecting signals having other frequencies. The United States cellular system (AMPS) divides the allocated spectrum into 30 KHz bandwidth channels and uses FM modulation.
Time division multiple access (TDMA) technology also utilizes narrow and discreet channels within the frequency band. However, each channel is further divided into time slots in the time domain. This results in multiple users on the same frequency channel and increases the number of users per given channel.
Unlike FDMA or TDMA, code division multiple access (CDMA) involves multiple users simultaneously sharing a channel having relatively wide bandwidth. CDMA standards (IS-95) currently specify a CDMA channel having a bandwidth of 1.25 MHz. In CDMA, a large number of signals share the same frequency spectrum. Each signal consists of a different psuedorandom binary sequence that modulates a carrier signal (at the center frequency of the channel""s spectrum). This spreads the spectrum of the waveform over the entire channel bandwidth. Use of CDMA technology allows for a larger number of signals than that used in FDMA or TDMA within the same amount of frequency spectrum.
Typically, service providers in PCS, cellular and other mobile applications divide the particular geographic region in which they are operating into xe2x80x9ccellsxe2x80x9d. This concept is well-known in the industry. Each cell contains a base station (including a transmitter and receiver) and services subscriber users within the boundaries of the cell. Each service provider is free to design its own coverage system including the locations and sizes of its cells, and this usually occurs. As a result, one cell of a first service provider may overlap completely or partially with a cell of another service provider. In FDMA and TDMA, adjacent cells must use channels having different frequencies to avoid interference. In CDMA technology, however, each cell may use all or any portion of the frequency spectrum allocated to the service provider.
All cellular systems were initially designed and deployed with FDMA or TDMA technology. Because no additional frequency spectrum has been allocated by the FCC, service providers desiring to use CDMA technology must utilize the same frequencies currently allocated. Using CDMA technology with FDMA or TDMA, in the current system, or use of CDMA in an all-CDMA system raises some interference problems. This interference is caused by out-of-band emission or intermodulation products of radio transmitters (both subscriber stations and base stations) of one service provider that are in or near the cell coverage area of another service provider.
As described earlier, each service provider within a particular geographic location may use either FDMA, TDMA (including GSM) or CDMA technology in its system and may use any number of cells and locations for its base stations in its system. This overlap increases the possibility of interference. Base stations and subscriber stations of a different technology or same technology operating at a different frequency band will produce out-of-band emission or intermodulation products. These out-of-band emission or intermodulation products may be strong enough to degrade the performance of a nearby channel that belongs to a different service provider. This problem is more acute when the two frequency bands are close to each other, such as adjacent or alternate bands. An example can be a GSM/TDMA/CDMA band A interference into CDMA band D (e.g. PCS) or CDMA band B (e.g. cellular), or an AMPS/TDMA/CDMA cellular band interference into CDMA cellular band.
One method of reducing interference in the cell of a first service provider that is caused by signals from subscriber stations and base stations of another service provider in or near the cell coverage area of the first service provider is to collocate the two base stations. Collocation occurs where each of the service providers locates their base stations for a particular coverage area at the same location (i.e. having their antenna""s at the same place, such as on top of the same building or tower). Such collocation reduces the interference between the nearby channel base stations and may ensure that the subscriber stations receive a strong signal in comparison to the interference. The interference will be much weaker than the desired signal resulting in negligible or no degradation in the performance of the communication channel.
However, the aforementioned method has several disadvantages. First, each service provider may not agree to collocate with the other user, especially if one provider is experiencing less interference than the other provider. Second, collocation may not be feasible. Space at a particular location may be limited or the owner of the space may not desire to lease space to the other provider. Most importantly, since the current system for each provider has already been designed and deployed, it may be quite expensive for one or the other service providers to redesign the locations and sizes of their cells and entire system. This may be an expensive solution, since it puts limit on where the base stations may be deployed.
Accordingly, there exists a need for a system that avoids interference with subscriber stations and a base station of a service provider caused by out-of-band emissions and intermodulation products from subscriber stations and a base station of another service provider having a different technology or same technology and operating at a different frequency band. Furthermore, there is a need for such a system that does not require collocation of the base stations of the service providers.
According to the present invention, there is provided a communications cell having a base station that emits a base station signal for communicating with one or more subscriber stations within a predetermined geographic area. The base station and the one or more subscriber stations communicate with each other via a communications channel. An RF repeater operable with the base station is located substantially proximate a second base station that is emitting interference in the communications channel which is received by a subscriber station located near the second base station. Use of the RF repeater reduces or lessens the effects of the interference in the communications channel.
In another aspect of the present invention, there is provided a communications cell including a base station for communicating with one or more subscriber stations within a predetermined geographic area. The base station and the one or more subscriber stations communicate with each other via a communications channel. An RF repeater located substantially proximate a second base station provides a communications link between the base station and a subscriber station located near the second base station. The subscriber station emits interference into a communications channel of the second base station. Use of the RF repeater reduces the effects of the interference on the second base station""s communications channel by reducing the transmit power of the subscriber station.
In yet another aspect of the present invention, there is provided a method of reducing or lessening the effects of interference on communications within a cell that is generated by an interference source located within or near the coverage area of the cell. A base station signal is transmitted from a base station located within the cell. The signal strength of base station signal at the location of a subscriber station near the interference source is such that the interference generated by the interference source interferes with communications directly between the subscriber station and the base station. An RF repeater located substantially proximate the interference source also receives the base station signal and re-transmits the base station signal for reception by the subscriber station located near the interference source. The signal strength of the base station signal transmitted from the RF repeater at the location of the subscriber station is sufficient to allow communication between the subscriber station and the base station via the RF repeater.
In another embodiment of the present invention there is provided a method for reducing the effects of interference on communications within a cell that is generated by an interference source located within or near the coverage area of the cell. The method includes the steps of transmitting a base station signal from a base station located within the cell, receiving the base station signal at an RF repeater, and transmitting the base station signal from the RF repeater to be received by a subscriber station located near the interference source. The base station signal transmitted from the RF repeater has a signal strength at the location of the subscriber station sufficient to allow communications between the subscriber station and the base station via the RF repeater. This reduce the effects of interference received by the subscriber station from the interference source.